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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 4 May 2011 by Riduna

Introduction

A target of 450 parts per million (ppm) CO2 in the atmosphere is widely regarded as synonymous with keeping mean global temperature by 2100 to no more than 2°C above pre-industrial levels. This is very misleading and dangerous. For reasons set out below, achievement of that target, probably by 2030, is likely to result in mean global temperatures dangerously in excess of the predicted 2°C.

At present we use the concentration of CO2 in the atmosphere as the indicative measure of future temperature. In doing so, we ignore the effect of other greenhouse gas emissions on temperature. Why? Because atmospheric concentration of CO2 is within our control andit is a widely endorsed IPCC finding that the cooling effect of aerosols and changed land use fully off-set the effect of other greenhouse gases.

That contention has become a convenient convention rather than a reality. It would be reality were the warming effects of greenhouse gas emissions and the cooling effects of aerosols relatively constant. But that is not so. We now know that, as a result of human activity, emissions of other greenhouse gases, particularly methane, are increasing while cooling off-sets are diminishing as countries reduce aerosol emissions to achieve cleaner air.

Methane

Although it has a relatively short residence in the atmosphere (~10 years) and low concentration, methane is a particularly dangerous greenhouse gas. It has much greater capacity to absorb and radiate long wave energy before oxidizing to CO2.

Although produced by “natural” processes such as decay of organic material, most methane is released into the atmosphere as a result of human activity. This includes methane produced by the massive numbers of farmed animals (cattle, chickens, pigs), coal and other mining, oil refining and, most noticeably, as a feedback from anthropogenic global warming. The latter is responsible for thawing of permafrost and methane clathrates.

Permafrost, particularly on polar land, has resulted in organic material located on or beneath the surface being frozen. That stopped the decay process and associated release of methane. Global warming, particularly polar amplification, is causing permafrost to melt, initially at the surface then to an increasing depth.

This has two effects. First, buildings and other structures located on frozen land are subject to damage and destruction as permafrost thaws. Second, organic material also thaws and resumes decaying, releasing methane and other gases in the process. The quantum of this material and methane produced from thawing of land based permafrost are not known but are thought to be significant. Emissions from this source are increasing, but do not pose as great a threat as thawing methane clathrate.

Methane continuously seeps from the earths crust. When it comes into contact with very cold water it forms an ice-like substance known as clathrate. Normally this substance is stable at depths of 360 meters (m) in the Arctic, though studies in the Svalbad region of northern Norway show that stability is now maintained at depths >400m, confirming the warming of Arctic Ocean waters.

When it melts, clathrate releases approximately 168 litres of methane for every litre of solid clathrate. It occurs beneath sediments offshore along the coastline of land bordering the Arctic ocean and has also been found in the Antarctic. When clathrate melts at depths <400m, methane bubbles to the surface and enters the atmosphere where over a period of 10-12 years it oxidizes to CO2. Clathrate melting at greater depths usually oxidises to CO2 before it reaches the surface, a process which creates hypoxic conditions, inimical to water breathing animals.

Shakhova (2010) reports that permafrost under the East Siberian Arctic Shelf, is, thawing and starting to leak large amounts of methane into the atmosphere and doing so at an accelerating rate. She estimates that 1.1 million tones of methane per annum now enters the atmosphere from this source, 3 times as much as is released from on-shore marshlands in this area. By 2030, those emissions are expected to reach 1.5 gigatonnes/annum.

Consequently, atmospheric presence of methane in the Arctic has now reached the highest it has been for >400,000 years, 1.85ppm compared to 0.7ppm normally found during warm periods. In parts of the East Siberian Arctic, methane in the atmosphere exceeds 2ppm. This is partly responsible for temperatures in the Arctic rising 2-3 times faster than in the tropics – the so-called Arctic amplification, expected to continue and accelerate the release of methane and ice melt.

Carbon Dioxide

Methane oxidizes to CO2 which has a residence in the atmosphere of ~100 years. In so doing it reduces the concentration of oxygen in the atmosphere and seawater. Amounts of CO2 entering the atmosphere from this source will increase as the rate of methane emissions increases.

Vegetation sequesters CO2 from the atmosphere and long-lived plants would normally retain it for several hundred years, possibly longer if the decaying process is arrested through development of anoxic conditions such as burial under sediment.

Rising global temperature is contributing to a large number of trees and other vegetation being killed by insect and other pathogen such asPhytophthora, as well as drought and fire. Trees which are burned release their stored CO2 immediately. Those killed by drought and infestation decompose more slowly and take longer to emit CO2.

As temperature continues to increase, damage to vegetation and release of CO2 will also increase. Forests previously thought of as carbon sinks are becoming a major source of carbon emissions. Damage to the Amazon rainforest by the 2010 drought is estimated to have caused reduced ability to absorb CO2 and increased emissions resulting in net release into the atmosphere of 2.2 gigatonnes of CO2. Drought events of this magnitude are expected to occur more frequently.

Over the last 20 years of curbing CFC emissions, damage to the ozone layer has been partly reversed and is expected to be fully recovered by 2050. Recovery will restore the radiative forcing of the ozone layer and have a warming effect globally and particularly over the Antarctic Region.

Aerosols

The radiative effects of aerosols are complex and short-lived but overall, they have a cooling effect. Increased industrial aerosol emissions 1940–1970 were probably responsible for mich of the slight global cooling trend which occurred in that period. Since 1970, industrialised nations have significantly reduced aerosol emissions through legislation, regulation and, following collapse of the Soviet Union, by closure of old, high emitting factories.

More recently, industrial aerosol emissions have been reduced by India and China, where they continue to pose a health problem. As a result, the cooling influence of aerosols has diminished and will continue to do so.

Conclusion

As global temperature rises it causes fast (loss of albedo) and slow (clathrate melting) feedbacks. A lot of work has been done on examining the effects of fast feedbacks – much less on slow ones, even though their effect on global temperature is becoming increasingly evident.

Slow feedbacks are increasing at an accelerating rate, as are their warming effects. Stratospheric ozone is increasing reversing the cooling effect caused by its loss, particularly over Antarctica. Cooling aerosols are diminishing as countries reduce their emissions because of their health effects.

Prior to these developments atmospheric CO2 concentration of 450ppm was equated as limiting average global temperature to 2°C above pre-industrial levels by 2100. This can no longer be maintained. Hansen and Sato (2011) using paleoclimate data rather than models of recent and expected climate change warn that “goals of limiting human made warming to 2°C and CO2 to 450 ppm are prescriptions for disaster”.

They predict that pursuit of those goals will result in an average global temperature exceeding those of the Eemian, producing decadal doubling of the rate polar ice loss, resulting in sea level rise of up to 5m by the end of this century. That prognosis is one which can not be ignored. Atmospheric CO2 concentration of 450 ppm may be an icon to which politicians and others cling but it is wrong and dangerously so.

Taking into account all of the above matters, what concentration of CO2 will limit global warming to less than 2°C above pre-industrial levels by 2100? Hansen suggests 350 ppm, warning that anything above this is dangerous.

Are you sure about this?
"In parts of the East Siberian Arctic, methane in the atmosphere exceeds 2ppm. This is partly responsible for temperatures in the Arctic rising 2-3 times faster than in the tropics"

The Zepplin station in Svalbard shows a slight decline in YOY methane concentrations.
http://www.esrl.noaa.gov/gmd/dv/iadv/graph.php?code=ZEP&program=ccgg&type=ts

I would have thought Arctic amplification is far more likely to be down to factors such as increased water vapor in the air, changes in ice albedo than methane.

I both agree and disagree with your position re the target of 450 ppm.

I agree that it is dangerous, and that the effect on the environment are likely to be significant.

However, I disagree that it should not be an aspirational target, because given the current state of politics and the pitiful efforts to reduce current emissions, 450 ppm is likely to be at the low end of what we can realistically achieve. I am very concerned by this, and I despair about what the state of the environment is going to be at the end of this century because of it. But what can you do?

Shakova and Semiletov 2007 reports: the surface layer of shelf water was supersaturated up to 2500% relative to the present average atmospheric methane content of 1.85 ppm, pointing to the rivers as a strong source of dissolved methane -- emphasis added

Why have you written an article about 450ppm CO2 when the issue is 450ppm CO2e? Off the cuff, CO2 is into 'the red zone' at about 425ppm with the rest coming primarily from increases in CH4.
Pre-Ind 2009 Increase Forcing wm2
C02 280 ppm 388 ppm 108 ppm 1.46
CO4 700 ppb 1745 ppb 1045 ppb 0.48

With CO2 concentrations growing at about 2ppm per year, it's less than 20 years to be 'in the red zone'. But even the 2 ppm growth rate will be surpassed once the current global recession gives way to new global growth.

At this point 400ppm, sustained over time is not a safe number- as Jim Hansen has said, while in Australia over the winter.....

In new research just out, Hansen concludes that at the current temperature, no “cushion” is left to avoid dangerous climate change, and that the Australian government target goals “… of limiting human-made warming to 2° and CO2 to 450 ppm are prescriptions for disaster”.

The question Hansen raises is direct and brutal in its implications: is the planet already entering a zone of dangerous climate change?

In a draft of a new research paper, Hansen and his collaborator Makiko Sato has opened a new debate about what might be the conditions for a safe climate; that is, one in which people and nations can continue to live where and as they have been, with secure food production, and in a bio-diverse environment.

The period of human settlement over the past 10,000 years is known as the Holocene, during which time temperatures and hence sea levels (the two having a close correspondence) have been remarkable stable. Temperatures over the period have not been more than 0.5C warmer or cooler than the mid-line (see chart). The warmest part of the Holocene (the “Holocene maximum”) was about 8000 years ago, and according to Hansen, today’s temperature is about, or slightly above, the Holocene maximum:

That is, we are already a little above the Holocene maximum. This matters because Hansen’s and Sato’s look at climate history (paleoclimatology) in this new research finds that it is around this temperature level that the large polar ice sheets start to behave differently. During the Holocene, the Greenland and Antarctic ice sheets have been relatively stable, as reflected in the stability of the sea level. But once substantial melting starts, the loss of heat-reflecting white sea-ice, which is replaced by heat-absorbing dark ocean water, produces an “albedo flip”:

Their conclusion is that:

“… the stability of sea level during the Holocene is a consequence of the fact that global temperature remained just below the level required to initiate the ‘albedo flip’ mechanism on Greenland and West Antarctica.”

The implication is clear that “just above” the Holocene maximum lurks real danger. As Hansen and Sato say:

“… the world today is on the verge of a level of global warming for which the equilibrium surface air temperature response on the ice sheets will exceed the global mean temperature increase by much more than a factor of two.”

To put it bluntly, we are on the edge of a precipice in terms of large ice-sheet losses and sea-level rises, and there is little “cushion” left:

“Polar warmth in prior inter-glacials and the Pliocene does not imply that a significant cushion remains between today’s climate and dangerous warming, rather that Earth today is poised to experience strong amplifying polar feedbacks in response to moderate additional warming.”

“… the fundamental issue is linearity versus non-linearity. Hansen argues that amplifying feedbacks make ice-sheet disintegration necessarily highly non-linear. In a non-linear problem, the most relevant number for projecting sea level rise is the doubling time for the rate of mass loss. Hansen suggested that a 10-year doubling time was plausible, pointing out that such a doubling time from a base of 1 mm per year ice sheet contribution to sea level in the decade 2005-2015 would lead to a cumulative 5-metre sea-level rise by 2095. “

Here Hansen repeats his view, first published in 2007 but widely ignored, that a 5-metre sea-level rise is possible. In fact, recent research by Blancon et al published in Nature in 2009, examining the paleoclimate record, shows sea-level rises of 3 metres in 50 years due to the rapid melting of ice sheets 123,000 years ago in the Eemian, when the energy imbalance in the climate system was less than that to which we are now subjecting the planet.

We are perhaps already a few tenths of a degree above the Holocene maximum, and the system seems to be in the early stages of rapid change. It is widely expected Arctic sea-ice will be totally lost in summer with a few years to a decade or so, perhaps at less than 1C or warming. Very few scientists think Greenland would be stable in an Arctic with little or no summer sea-ice, and opinion is split as to whether it is past its tipping point already.

It is hard to argue that anything above the Holocene maximum (of about 0.5 degrees above the pre-industrial temperature) can preserve a safe climate, and that we have already gone too far. The notion that 1.5C is a safe target is out the window, and even 1 degree looks like an unacceptably high risk.

owl905 wrote: "Why have you written an article about 450ppm CO2 when the issue is 450ppm CO2e?"

Which... is one of the points made by the article.

As the article notes in the second paragraph, CO2 ppm alone is often used on the grounds that aerosol cooling offsets warming from other greenhouse gases. However, as the remainder of the article then explains, warming from other greenhouse gases is increasing while aerosol cooling is decreasing.

It will be interesting to see what sort of limits IPCC 5 suggests. Given that ice loss and sea level rise are progressing much faster than previously estimated I have to wonder if the 2 C 'safe' increase itself isn't out the window... regardless of how we get there. newscrusader's post above makes the same point in greater detail.

That said, this may actually be 'good' news in a way. Yes, we have probably already gone beyond the point where we are going to significantly raise sea levels and have planet-wide changes to weather patterns for thousands of years... but this is happening fast enough that it MAY help to wake people up to reality before we get to the point where we vastly decrease agricultural production and/or devastate the ocean ecosystem.

I don't know where to really post this, but, not being a scientist, can someone explain how CO2, being heavier than air, can be consoidered a greenhouse gas. Also much is said about CO2 emissions from motor vehicles. How is it then, if one decides to commit suicide in a car, that it is CO poisoning that is the cause of death.

RAYJ, to quickly answer the easiest part of your comment (the last line), Carbon Monoxide (CO)is produced from the partial oxidation of carbon-containing compounds; it forms when there is not enough oxygen to produce carbon dioxide (CO2)AlsoExposures at 100 ppm or greater can be dangerous to human health.

Carbon dioxide (CO2)is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state. CO2 is a trace gas comprising 0.039% of the atmosphere.
1% (10,000 ppm) will make some people feel drowsy. Concentrations of 7% to 10% [70-100,000 ppm] cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.

But since it's such a simple question, concerning CO2 being heavier than air... the speeds of molecules in a gas are so great that their relative masses are inconsequential when it comes to mixing. There are much more massive molecules in the atmosphere (Argon, atomic weight 40, Krypton, atomic weight 84) and you don't see them all sinking down and blanketing the surface.

Concerning CO and CO2, CO is directly poisonous to most living things because it binds directly to hemoglobin and interferes with the absorption of oxygen the blood stream.

CO2 is not a poison. It is, however, a greenhouse gas which affects climate. The fact that CO2 is not poisonous is not a rational argument for saying that it is therefore harmless.

I regularly read different climate blogs. Until now, I was under the impression that predictions for a sea level rise by the end of this century were between 0.5 and 2m. That Hansen should predict 5m and so soon (by 2095!! not 2195 or 3095) surprises me.

Do you know what the reaction to Hansen is among climate experts (the kind that publish peer reviewed papers on climate change)? Is this a consensus view?

Melting permafrost - is obviously a source of methane. But also sink for around 25% CO2. We do not know what will be the balance sheet.

... However:
Melting permafrost is an expanding area of “waterlogged habitats” - a wetlands.
And wetlands ... Carbon accumulation in soils of forest and bog ecosystems of southern Valdai in the Holocene, Minaeva et al. 2011.: “The results show that carbon stocks in mineral soils are many times smaller than in waterlogged soils and an order of magnitude smaller than in bog soils. Mineral and bog soils are characterized by similar rates of carbon accumulation averaged over the entire period of their existence. The highest rate of carbon accumulation has been noted for the soils of waterlogged habitats, although this process may be periodically disturbed by fires and other stress influences.”

Rising global temperature is contributing to a large number of trees and other vegetation being killed by insect and other pathogen such as Phytophthora, as well as drought and fire.

That's not true - diseases and pests have a their enemies and they - usually - limited - strongly - cooling.

“Arnone et al. (2011) experimentally imposed an extremely warm year on intact tallgrass prairie monoliths from Oklahoma, USA, and found an immediate and large (~30%) reduction in above-ground productivity, driven primarily by the dominant C4 grass species in the system. However, the change in above-ground productivity was well within the range of interannual variability observed for tallgrass prairie, was not persistent over time, and was not accompanied by changes in plant community structure or composition. Similarly, for intact tallgrass prairie in north-eastern Kansas, USA, Smith (2011) shows that over a decade of experimentally imposing an extreme precipitation regime, whereby the number and timing of rainfall events were altered well beyond historical patterns, the resulting reduction in above-ground productivity, although ecologically significant, also fell within the natural range of variation for the system and was not accompanied by substantive changes in the plant community. Finally, with a long-term field experiment where statistically extreme drought events were applied to constructed European grassland communities for five consecutive years, (Jentsch et al. 2011) show a lack of large effects for the majority of the 32 response parameters measured. For example, above- and below-ground productivity remained unchanged across all years of the study ...”

Martin,
Hansen is at the upper level of scientific predictions. His is not a consensus view (yet). On the other hand, the consensus estimate for sea level rise has gone up about a meter in the last five years. Hansen has generally had high predictions in the past relative to the consensus view. He has often been correct. Hopefully he will be wrong this time.

@muoncounter 5
Also is there any evidence that the imbalance in methane is unusual? With the bulk of the large continents on the Nothern Hemisphere that plus agriculture and industry we would expect a NH bias in concentrations of such a transient gas would we not?

Also the graph shows a difference of around 10% in methane concentrations so not really enough to form a major part of arctic amplification which is abotut 2 to 3 times the warming rate of the rest of the world.

By what we know now, we can neither consider the 450 ppm nor the 2 deg targets as acceptable, risk-management-wise. Let us by all means hope that Hansen is much more wrong this time than he has, on the average, been in the past, but we just can't count on that.

I tend to agree with CBD (#9) that this time, changes may happen fast enough to make us react. Because of arctic amplification and weak negative feedbacks, arctic melting may enter an exponential phase, which soon enough will make itself felt world-wide. We are just in the beginning of such a phase (at worst), but when the process is established, forecasts bases on linear or polynomial extrapolations soon become short-lived.

This post should be labeled "speculative". It is contradicted by the more recent post by dana1981 which points to the HK08 paper stating that 450 ppm will result in 2C of warming at equilibrium (i.e., less warming sooner than equilibrium) using their best (3C) estimate of sensitivity. This paper http://escholarship.org/uc/item/1hq91854.pdf points out several facts about methane not mentioned above, such as the fact that the clathrates are stable below 440m in the Gulf of Mexico (a long way from the Arctic) and that there is a huge amount uncertainty in the estimates of clathrate stability hence the label of speculative for this post.

Eric, I would also have to agree with you. Also note that article at realclimate and the papers referenced therein. While I recognize that methane is a risk(think PETM), so far there isnt that much evidence for getting seriously worried.

CB Dunkerson quoted & wrote:- "owl905 wrote: "Why have you written an article about 450ppm CO2 when the issue is 450ppm CO2e?"

Which... is one of the points made by the article."

No it isn't; not in the least. The article is pointing at the 450 mark for CO2 and adding subjective inclusions from the rest of the pollution problem.

"As the article notes in the second paragraph, CO2 ppm alone is often used"

No it isn't. That's the point of my objection - the 'often used' is CO2e - and that's the correct framework.

"... on the grounds that aerosol cooling offsets warming from other greenhouse gases."

There's no such grounds, no such basis, and no science claiming a past equilibrium that extrapolates to a 'CO2/450ppm.' The statement appears to be a misinterpretation of the Radiative Forcing historical chart used by the IPCC (AR4-WG1-Ch2, p.136).

From the article:

"At present we use the concentration of CO2 in the atmosphere as the indicative measure of future temperature."

Untrue. CO2e is used literally everywhere. The CO2 focus isn't because of a preeminence in temperature rise, it's because of the long residency in the source/sink cycle.

When Hansen and Sato 2011 get trough peer review (which I think it won't), I'll take it seriously.

5 m. sea level rise caused by 450-ppm CO2 concentration is far from being not only consensus but taken seriously by thier colleagues. I don't even think that "more than 1 m." can be regarded as consensus, given that it's not suppoted by physical models.

Now the AMAP assessment finds that Greenland was losing ice in the 2004-2009 period four times faster than in 1995-2000.

That’s a doubling time of about five years, a continuation of which terminates the ice sheet by about the 2060s. This is exactly what Hansen was worried about in his recent paper. It’s true that there’s no paleo-analog for such a rapid collapse, but similarly there’s no paleo-analog for the unnatural forcing we’re applying. The West Antarctic Ice Sheet would disappear at least as quickly, and if both go that’s about 13 meters sea level rise. Continued thermal expansion, a relatively modest contribution from the East Antarctic Ice Sheet and minor sources would likely push things into the 20 meter range.

How safe is 450 ppm CO2? Think of sea level. How high was it the last time CO2 went above 440? How high can the sea rise by 2100? Given that the great, massive ice sheets will slip into the sea faster as they warm, and that the process can hardly be expected to be merely linear, + 5 meters can not be ruled out even if it is the high end of predictions. Anything over + 1.5 meters is a very large problem. How quickly can you move a large city? Where would you put a whole bunch of them in a hurry?

Policy implications: first note that we are on course to far exceed 450 ppm by 2100, and time does not stop then. To sanely set policy, one must first bound the risk, or in other words determine a worst case scenario. Then set policy to allow virtually no chance for that case.

Conclusion: stop burning carbon. We have other energy sources, we only lack a decision to use them.

00

Moderator Response: [DB] 1.5 meters is a fraction of Miami, but about 90% of its high-end real estate.

#23, newscrusader
"Now the AMAP assessment finds that Greenland was losing ice in the 2004-2009 period four times faster than in 1995-2000."

There are conflicting data on the rate of Greenland ice loss, and if the ice sheet was closer to balance in 1990-95, a fourfold increase does not have to be very significant towards very rapid meltdown. Four times very small is still very small.

It is only when we have reached a much higher rate of ice loss than today, and it keeps on multiplying, that we can start talking about a rapid meltdown. And we are definitely not there yet. But I can't see how anyone can be so sure we won't get there, in some not too distant future.

00

Response:

[DB] As an FYI, the dangers to changes in SLR from Green Ice Sheet mass loss lie less in surface melt, which is still considerable, than they are due to increases in calving and ice stream transport in marine terminating glaciers like Jacobshavn, Petermann and Zacharaie.

The latter, Zacharaie, is the one with the most potential to change from its current transport and calving dynamic into a different dynamic/phase state of greatly increased transport and calving.

Pete Dunkelberg, My undestanding is that "over 1 meter" projectons come from semi-empirical approaches (pioneered by Rahmstorf), but physical models (those used by the IPCC) don't project such high rises.

Semi-empiricals may perfectly be right, but I woudln't call it a consensus yet. In any case, Hansen & Sato are way off the numbers published in the peer reviewed literature. They count on Cenozoic big ice sheets such as Laurentide that aren't here any more. And I don't really see any scientist supporting their high estimates ever (the same when suggesting that sensitivity is 6ºC). In my view, RealClimate silence about this (as previously with Hansen's Venus syndrome) si somewhat telling.

Here is the link to Hansens paper concerning sea level rise.
http://www.columbia.edu/~jeh1/mailings/2011/20110118_MilankovicPaper.pdf
Scroll down to page 15 to see the graph where he argues that a 10 year doubling of the melt rate for Arctic and Antarctic ice is possible. It can be argued that a 7 year doubling is possible.
Time will tell.

Alexandre, Rahmstorf & Vermeer base their projections on empirical correlation between 20th Century SLR and temperature. However, 20th Century SLR doesn't have a big contribution from ice sheets (thermal expansion plays a big role), so I don't think it can reflect future contribution from ice sheets. I think that warmer deep ocean would be a more likely explanation of the IPCC undersetimation (and this would hopefully help to close the global heat budget).

On the other hand, the IPCC projections do include a contribution from increased ice flow from Greenland and Antarctica at the rates observed for 1993-2003. The point by Rahmstorf and Vermeer being that it may accelerate.

My comment was motivated by Martin #13:

"[...] Until now, I was under the impression that predictions for a sea level rise by the end of this century were between 0.5 and 2m. That Hansen should predict 5m and so soon (by 2095!! not 2195 or 3095) surprises me.

Do you know what the reaction to Hansen is among climate experts (the kind that publish peer reviewed papers on climate change)? Is this a consensus view?"

I'm trying to answer that by saying that this projection by Hansen & Sato cannot be considered anything close to a scientific consensus (besides, it's not peer reviewed and it's quite different to what peer reviewed papers say), and highlight that even those latest figures we are recently more used to (+1 meter) must be taken with some caution, as we always look for physical expalantions, rather than empirical correlations. Yes, I do know that current data show that IPCC projections are undersetimating SLR, but between 19-59 cm (IPCC) and more than 1 meter there's a big gap. SLR is, of course, a serious threat with any of those numbers.

Sphaerica, regarding the citation, I'm trusting this blog post:

"Hansen and Sato (2011) using paleoclimate data rather than models of recent and expected climate change warn that “goals of limiting human made warming to 2°C and CO2 to 450 ppm are prescriptions for disaster”.

They predict that pursuit of those goals will result in an average global temperature exceeding those of the Eemian, producing decadal doubling of the rate polar ice loss, resulting in sea level rise of up to 5m by the end of this century."

I was expecting a link to a RealClimate blog post. You did not provide a link, and a Google search shows no hits anywhere for the text that you have posted.

A search for "sea level" rise at realclimate.org, however, provides any number of reasoned, scientific posts at RC explaining upper and lower boundaries for sea level rise, and the reasons behind each. In particular, reasonable estimates recognized by RC fall between 0.5 and 2.0 meters.

I see nothing remotely close to a comment at RC suggesting that a 5m rise by 2100 is at all likely, and must insist that if you cannot produce a link to such a statement, you must openly and loudly withdraw the statement that "RealClimate silence about this ... is somewhat telling".

From the Hansen and Sato 2011 paper you posted:

Alley (2010) reviewed projections of sea level rise by 2100, showing several clustered around 1 m and one outlier at 5 m, all of which he approximated as linear. The 5 m estimate is what Hansen (2007) suggested was possible, given the assumption of a typical IPCC's BAU climate forcing scenario.

Also:

However, the fundamental issue is linearity versus non-linearity. Hansen (2005, 2007) argues that amplifying feedbacks make ice sheet disintegration necessarily highly non-linear. In a non-linear problem, the most relevant number for projecting sea level rise is the doubling time for the rate of mass loss. Hansen (2007) suggested that a 10-year doubling time was plausible, pointing out that such a doubling time from a base of 1 mm per year ice sheet contribution to sea level in the decade 2005-2015 would lead to a cumulative 5 m sea level rise by 2095.

Non-linear ice sheet disintegration can be slowed by negative feedbacks. Pfeffer et al. (2008) argue that kinematic constraints make sea level rise of more than 2 m this century physically untenable, and they contend that such a magnitude could occur only if all variables quickly accelerate to extremely high limits. They conclude that more plausible but still accelerated conditions could lead to sea level rise of 80 cm by 2100.

You will note that Hansen is not with this projecting a 5m rise by 2095. He's doing math. He's saying if ice sheet disintegration is non-linear, and if doubling time for rate of mass loss is the best predictive factor, and if that doubling rate is 10 years (which is plausible, but in no way predicted), then the math says that a doubling of 1 mm/year from 2005-2015 would arrive at a cumulative 5 m sea level rise by 2095.

He never says this is going to happen. He never suggests that it will. The paper in many places is actually a rather dispassionate discussion of all of the various estimates of others (as well as Hansen 2007) on sea level rise.

From Hansen's 2007 paper, where the proposal was presented (given that Hansen and Sato 2011 is merely summarizing the current state of the literature):

Of course I cannot prove that my choice of a ten-year doubling time for nonlinear response is accurate, but I am confident that it provides a far better estimate than a linear response for the ice sheet component of sea level rise under BAU forcing.

His point is clearly that linear estimates are overly simplistic, and that a non-linear mechanism could produce a dangerously higher value. The point here is not the value. It's not a prediction. It's a demonstration of the importance of not taking a purely linear approach to a non-linear problem, much in the way that exponential growth is taught to school children by pointing out that if they start with a penny, and double it every day, by the end of a month they have over 10 million dollars.

From the same paper, a few sentences later:

The nonlinearity of the ice sheet problem makes it impossible to accurately predict the sea level change on a specific date. However, as a physicist, I find it almost inconceivable that BAU climate change would not yield a sea level change of the order of meters on the century timescale.

Suggestion:If something bothers you, take the time to actually read the source material, in the proper context and perspective, and pay attention to the words, not your emotional reaction to the words.

As I understand it (feel free to correct me), this is the main cause of the IPCC underestimate.

I agree that, at least at this point, the 5m SRL projection for 2100 is a big outlier. OTOH I don't know how far a consensus goes here, but I thought the 1+ m was already considered to be quite plausible at the BAU scenario.

Sphaerica, I think there's some misunderstanding here. I just said that RC didn't say anything about Hansen & Sato's 20th century multimeter [if you like it more than 5m] SLR suggestion. Of course I cannot link to silence, because it just means they've ignored it. The citations you provide are, of course, previous to Hansen & Sato 2011 and I think they just support my point: that Hansen & Sato's suggestion of 5m SLR is not supported by any other scientist.

Caution when assuming more than 1 meter SLR is my personal opinion, which I've tried to back up whithout mentioning RC at all.

I cannot imagine where you get the idea that I've said that there's any "comment at RC suggesting that a 5m rise by 2100 is at all likely" nor what makes you think I should provide such a link.

On the other hand, if you think Hansen made no such a projection of 5m SLR, then you should go against this blog post summary, not against my comments. In any case, my comments apply the same to the "I find it almost inconceivable that BAU climate change would not yield a sea level change of the order of meters on the century timescale". So now you seem more interested in discussing the semantics of "projection". Sorry I'm not. Regardless of the label of your choice for Hansen & Sato's multimeter fantasy, it's way off the peer reviewed literature numbers and they provide weak evidence (I'd be more open to the idea if they meant in something like 300 years rather than one century). As I said before, if they can get that through peer review, I will take it more seriously.

Regarding scientists' opinions on Hansen 2007, you can see William Connolley here or here, or James Annan here.

Alexandre, yes, I refer to the same IPCC projection. See your link where they say: "They include a contribution from increased Greenland and Antarctic ice flow at the rates observed for 1993-2003, but this could increase or decrease in the future".

I don't think we know what the cause of the IPCC underestimate is (I may be wrong, it's just my impression from what I've read in blog posts), but I guess it can only be ice sheets or thermal expansion. Given that thermal expansion seems easier to calculate and there's a lot of uncertainty about ice sheet dynamics, the latter turns up as a more likely culprit. However, I think this is rather speculative, especially considering this is a short-term comparison (we have just a couple of decades of data to compare with projections). See, for example,Deep ocean warming solves the sea level puzzle about Song & Colber 2011.

Don't trust me on the consensus matter, I'm not an expert, but that's my impression. I think the +1m is rather based on empirical data, which is quite compelling, but, lacking a physical understanding of the underlying causes, I think it's difficult to call it a consensus yet. Anyway, I may be too influenced by this discussion I had with Zorita and its later blog post.

This is B.S. You can't attack someone because they failed to say what you want to hear. RC has many, many posts on sea level. They're all rather objective and thorough. Their "failure" to explicitly come out and attack what was never even a projection is hardly an offense.

Hansen & Sato's suggestion of 5m SLR is not supported by any other scientist.

Perhaps because it doesn't exist? Where in the paper do you see this stated?

And if you haven't looked yourself, then why are you talking about it, and linking to the paper, when you haven't read it yourself?

Caution when assuming more than 1 meter SLR is my personal opinion...

The key words there being "my personal opinion."

...then you should go against this blog post summary, not against my comments

But you're spreading it as if it's true, without proper citation, and without even having followed up yourself. In fact, you accused RC of "silence" without evidence, and you've accused Hansen and Sato of making a prediction which they didn't make.

And yet the fault isn't yours, you were just blindly parroting what you read on some blog somewhere (and yet you can't provide a link to what you read, and a Google search finds nothing). We can hardly blame you, now, can we?

...now you seem more interested in discussing the semantics...

No, I'm more interested in paying attention to what people are actually saying, instead of distorting their position to create a strawman. Your desire to equate "meters" with "5 meters" goes hand in hand with your own misunderstanding of what is being presented. Once again, you are taking the BAU comment out of context, and exaggerating what it says. Hansen's basic point is that sea level rise is unlikely to be linear, and most linear estimates wind up at 1m or 2m, meaning any non-linear increase must be over 2m, and therefore on the order of "meters."

This is reasonably well supported by the fact that ice loss is accelerating. Is it guaranteed? No. Is it a viable position? Yes.

But in the end, you attack RC, you attack Hansen, and you come back with bluster and hand waving. You can't support your statements, you won't even take the blame (it's the blogger's fault), but you still dig in and refuse to budge.

"you were just blindly parroting what you read on some blog somewhere (and yet you can't provide a link to what you read, and a Google search finds nothing)"

As I've already told you before, I've read it here, Sphaerica, in "this blog post" you and I are supposed to be commenting on; you can scroll up and read it. Do you want me to link to the web page you are already reading and commenting on?

I'm NOT atacking RC at all, quite on the contrary, they are the reference regarding climate change, and what I've said is that (for that same reason) the fact that they've just ignored this paper by Hansen & Sato (and Hansen 2007) on SLR suggests that it has no merit whatsoever (I'm with RC, not atacking them). Scientists as James Annan and William Connolley see it this way and say everybody else does (linked above). Your citations from RC on SLR are far from contradicting this. My point (from the very beginning) is that Hansen hasn't been able to persuade any scientist that more than 2 m. (up to 5m!) SLR in this century is a credible possibility. Your verbosity hasn't shown otherwise.

"BAU scenarios result in global warming of the order of 3-6°C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain.

The 5 m estimate is what Hansen (2007) suggested was possible, given the assumption of a typical IPCC's BAU climate forcing scenario."

Apologies. I misunderstood what you had written, thinking that "this blog" referred to whatever blog you were quoting, instead of "this blog" meaning Agnostic's post here. The confusion was entirely mine, and I apologize (although .

On the issue of RC, however, I'd point out that they have pretty much posted everything short of an actual, explicit, unequivocal refutation of Hansen's 2007 statement.

Again, let me point to one line from the Sept 2008 post (emphasis mine):

We stress that no-one (and we mean no-one) has published an informed estimate of more than 2 meters of sea level rise by 2100.

I'm pretty sure that the crew at RC has read Hansen 2007, and maybe spoken with him from time to time at lunch, so I think it's pretty safe to say that this statement includes Hansen 2007, and that RC has not been silent on this.

They later state:

The nearest thing I can find is Jim Hansen who states that “it [is] almost inconceivable that BAU climate change would not yield a sea level change of the order of meters on the century timescale”. But that is neither a specific prediction for 2100, nor necessarily one that is out of line with the Pfeffer et al’s bounds.

Thus, this media reporting stands as a classic example of how scientists get caught up trying to counter supposed myths but end up perpetuating others, and miss an opportunity to actually educate the public.

And from the August 2009 post:

And there are arguments (e.g. by Jim Hansen) that over time the ice loss may be faster than the linear approach suggests, once the ice gets wet and soft and starts sliding.

Again, I would argue that his statement and his logic is being woefully misinterpreted. It is not a prediction of 5 meter sea level rise, it is an explanation as to why 1-meter sea level rise and lower is unlikely, and a warning against putting too much emphasis on linear sea level rise projections.

Hansen admits that he has no firm ground for a real prediction, and so he doesn't make one: he restricts his statement to the more vague "meters" meaning more than 1 and less than a whole lot.

I think you should certainly correct your post. You present the 5m "prediction" from Hansen and Sato 2011 as if it is such, when it is nothing more than an extreme upper limit... and an outlier, much as the upper limit on warming set by the IPCC is around 9.5˚C.

Statements like yours can very easily be misinterpreted and result in a wild flurry of useless activity (as Jesus and I have done). In my book, it does qualify as alarmist, in that it presents the extremes of the science without clearly explaining the logic, or clarifying the middle ground (i.e. 0.8 to 2m sea level rise by 2100).

Again, the actual, relevant text from Hansen and Sato 2011:

Alley (2010) reviewed projections of sea level rise by 2100, showing several clustered around 1 m and one outlier at 5 m, all of which he approximated as linear. The 5 m estimate is what Hansen (2007) suggested was possible, given the assumption of a typical IPCC's BAU climate forcing scenario. Alley's graph is comforting, making the suggestion of a possible 5 m sea level rise seem to be an improbable outlier, because, in addition to disagreeing with all other projections, a half-meter sea level rise in the next 10 years is preposterous.

However, the fundamental issue is linearity versus non-linearity. Hansen (2005, 2007) argues that amplifying feedbacks make ice sheet disintegration necessarily highly non-linear. In a non-linear problem, the most relevant number for projecting sea level rise is the doubling time for the rate of mass loss. Hansen (2007) suggested that a 10-year doubling time was plausible, pointing out that such a doubling time from a base of 1 mm per year ice sheet contribution to sea level in the decade 2005-2015 would lead to a cumulative 5 m sea level rise by 2095.

Non-linear ice sheet disintegration can be slowed by negative feedbacks. Pfeffer et al. (2008) argue that kinematic constraints make sea level rise of more than 2 m this century physically untenable, and they contend that such a magnitude could occur only if all variables quickly accelerate to extremely high limits. They conclude that more plausible but still accelerated conditions could lead to sea level rise of 80 cm by 2100.

I'd also point out that Hansen's Eemian statements are far less dramatic than is implied by the conclusion of this post. There are two important points there. The first is that the peak Eemian temperatures were only 1˚C above those of the Holocene, so reaching them is not going to be all that hard given current estimates of climate sensitivity.

A second important point, relevant to sea level rise, is that sea levels during the Eemian were 5m higher than today, another indicator that if we allow temperatures to reach those levels, then at some point (not at all necessarily by 2100) sea levels could well reach those same heights.

having followed your link to Hansen's paper and read it, I have a few questions which you might be able to answer.
Hansen makes a number of statements supporting his hypothesis that a sea level rise of 5m by the end of this centurey is plausible.

Which of these statements is not close to the climate science mainstream?

1. The sea level rise is accelerating.
2. The increase in sea level rise is exponential.
3. The ice mass change and the ice mass change rate of Greenland and the Antarctic are compatible with a doubling period of 10 years.
4. Ocean cores are a much better guide to global mean temperatures than ice cores.
5. The Eemian and the Holsteinian interclacials were less than 1°C (probably only a few tenths) warmer than the peak holocene global temperature.

I'd point out that Hansen suggests that in a simple model sea level rise would be geometric, not exponential (although I made the same mistake in an earlier comment, saying "exponential" when I'd meant "geometric").

That is, he argues that if rate of ice mass loss is doubling every ten years, then sea level rise could double every ten years.

More importantly, however, we are at the very start of any ice melting, so there would as yet be no measurable geometric increase in sea level rise.

Given this, your statement #2 above would not be close to the climate science mainstream... but that's not relevant, and that's some of Hansen's point. The question isn't what's happened until now, the question is what will happen. His main point is that if melt rates are not linear, then sea level rise will not be linear, even if sea level increases seen to date are linear.

Your statement #3 is also invalid as stated. It is true that the rate of loss of ice in Greenland appears to have doubled in the past ten years. It is not valid to instantly extend this to say that this trend will continue without variation.

In addition, some have questioned the proper interpretation of the GRACE satellite data (specifically, the impact of isostatic rebound on those measurements), so even the exact measure of loss is not yet certain.

I think the rate of ice mass change in Greenland and the Antarctic is still in an early "wait and see" mode.

Martin, concerning ocean cores... I've certainly never seen the statement that ocean cores are a "much better guide to global mean temperatures than ice cores" anywhere. It's not in Hansen and Sato 2011. I'm not sure it's a question that scientists debate. They are different tools, with different qualities, and they are used as such and interpreted as needed.

From Hansen and Sato 2011:

Ice cores and ocean cores are complementary tools for understanding, together providing a more quantitative assessment of the dangerous level of human interference with the atmosphere and climate.

and later

Ice core and ocean core records each have limitations as a measure of global temperature. Here we point out constraints on both records and hypothesize a reason why these two records seem to differ during recent interglacial periods.

This is followed by a detailed discussion of the advantages and disadvantages of each.

And, concerning their impact on estimates of temperatures in the Eemian:

Ice cores and ocean cores are valuable complementary sources of climate information. Fig. 4 shows that they provide similar pictures of Milankovic glacial cycles, with one exception. Ice cores suggest that the Eemian and Holsteinian interglacials were warmer than the Holocene by 2°C or more. In contrast, ocean cores suggest that these earlier interglacials were warmer than the Holocene by at most one degree, perhaps by only tenths of a degree Celsius.

Of course, this is just from Hansen and Sato 2011. Your question is about the "climate science mainstream"... which I'm not sure is something that exists.

Sorry for repeating, but I want to make it clear that I didn't mean to criticise RC for being silent, I was rather interested in RC's opinion and I thought their silence was also telling. That's because I think the possibility of 2-5 m. SLR in this century suggested by Hansen & Sato is important enough as to devote an entire post to it, especially when they've been posting regularly on the subject. However, in the absence of a specific RC blog post, you've digged deeper than I to give some insight into RC's opinion.

Firstly, they seem to place Hansen's numbers within the bounds of Pfeffer et al. I do not go along with that. Pfeffer et al says 2 m. is their highest conceivable bound, but they find it highly unlikely and they add that "more plausible but still accelerated conditions could lead to sea level rise of 80 cm by 2100" (my bold).

Hansen and Sato says that under BAU scenarios, multi-meter SLR "are not only possible, but almost dead certain" and that 5 m is possible [citations at the end of comment #38]

I don't think 1.5 m is "multi-meter". I think that's downgrading the meaning of "multi". I think multi-meter means at lest 2 (and I think that's what most readers are interpreting, especially that was the reading of the comment I meant to reply to). I think Hansen tries to add something to the literature, and I think it's the suggestion that all current projections may be too low. So not only I don't think Pfeffer and Hansen are in the same bounds, but I think they're just opposite.

Then we move on to RC's relevant statement:

"We stress that no-one (and we mean no-one) has published an informed estimate of more than 2 meters of sea level rise by 2100."

Agreed. That's why I think RC is de facto rejecting Hansen & Sato's suggestion. In any case, it doesn't matter, what is important, in my view, is that, when talking about Hansen & Sato SLR numbers, we stress that the most widely accepted projections for 20th century SLR cluster around 1 m.

I think Hansen tries to add something to the literature, and I think it's the suggestion that all current projections may be too low.

Yes, absolutely, and I get the impression that the 2011 paper is intended to provide further weight to this argument, through paleoclimate evidence (i.e. comparison to Eemian temperatures and sea levels, without reference to time frame).

I think I separate from you in that I think that Hansen, and RC's interpretation of Hansen, is that he never actually made any specific projections, and certainly not within any particular time frame. He is pushing the idea that sea level rise will be non-linear, and in the simplest model for that (ice loss doubling every 10 years) this would project to a 5m increase by 2095... but he's not putting that out there as a serious prediction, only as a stark example of the potential difference between linear and non-linear.

I think that since then he has gone out of his way to avoid being specific. He is pushing the idea of a non-linear sea level rise without making any specific predictions relative to time-frame.

I think RC understands this thoroughly, which is why they appear to be silent. They don't refute 5m because they don't feel any such prediction was ever made. They don't refute "meters" because it is an unbounded, non-specific warning.

I don't think 1.5 m is "multi-meter"

I don't either. Without coming out and saying it, I think Hansen is implying 2m-5m, with the idea that 2m is at the low end, and anyone projecting below that is in for a surprise... but he's never come out and said that... this is my interpretation.

I think that's what most readers are interpreting...

No, I think they're taking away 5m because that's what the post above says, and it should be rectified ASAP, because it's going to confuse people.

what is important, in my view, is that, when talking about Hansen & Sato SLR numbers, we stress that the most widely accepted projections for 20th century SLR cluster around 1 m.

Agreed (except I know you meant 21st, not 20th, century), as long as people understand that those numbers presume a linear increase, and there are reasonably strong arguments to at least consider that that may not be the case, and we may be underestimating the problem (for the second time).

I think the last bit from Hansen and Sato 2011 worth pointing out is this:

We conclude that available data for the ice sheet mass change are consistent with our expectation of a non-linear response, but the data record is too short and uncertain to allow quantitative assessment. The opportunity for assessment will rapidly improve in coming years if high-precision gravity measurements are continued.

Finally, we note the existence of a strong negative feedback described by Hansen (2009) that comes into play when the rate of sea level rise approaches the order of a meter per decade. Such an iceberg discharge rate temporarily overwhelms greenhouse warming, cooling high latitude atmosphere and ocean mixed layer below current levels. Ice sheet mass loss may slow in response to this cooling...

That last bit, if it ever comes to past (certainly not in our lifetimes, I'm sure) is a downright scary image.

#43, Spherica
sorry, I clicked on the wrong button before I was finished.

That bit starting with 5. Interglacial ... is a quotation.
And so is this bit:
"We conclude that ocean cores provide a better measure of global temperature change than ice cores during those interglacial periods that were warmer than the pre-industrial Holocene."

Again my question, is this preference for ocean cores during warm interglacials main stream climate science or particular to Hansen and Sato.

I do not agree with picking a number such as 450ppm and declaring <450 is safe and >450 is dangerous.

I do agree with Hansen that 350 is much more like a reasonable 'safe' number, but rather than consider that we are already in 'dangerous' territory, and risk some people concluding 'OMG we are ruined', I would prefer a more gradual approach, and consider that above 350 is 'risky' rather than 'dangerous'.

By this I expect that for levels of warming up to about 2 degrees things will probably turn out quite ok. There will be some problems, but also some benefits, and nothing that we can't solve. But it is still not safe as we don't know for sure what can happen and there is always the chance something will go unexpectedly wrong and blow up bi time in our face (abrupt climate change, severe weather, methane clathrates etc)

Like cigarettes. A few cigarettes probably won't kill you. But its just not safe and you want to reduce the number of cigarettes as much as possible as soon as possible. And trying to find a dangerous level foor Co2 is like stating that smoking more than 10k in a lifetime is dangerous, and getting all depressed because you've already smoked 12k so why bother quitting....

(and of course not like cigarrets because Co2 has benefits and is absolutely required for life in small amounts)

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